The blades of gas turbine engines are arranged with a minimum clearance between the tips of the blades and the casings associated therewith, as any gap therebetween will contribute to a reduction in efficiency. Therefore, to minimise the clearance between the blade and the casing, titanium fan blades currently used may cut into a casing liner using the plain square edge of the tip. During the cutting or rubbing of the liner, the blade tip typically reaches temperatures of 500° C. and, for a titanium blade, such temperatures do not damage the tip. Further, the inherent strength of a titanium tip, where the blade is solid, allows the tip to survive a heavy rub or significant incursion into the liner, for example following a bird strike in a jet engine.
However, in the case of a composite fan blade, tolerating the high temperatures and surviving a heavy rub represent a significant engineering challenge. For example, the maximum allowable temperature for a carbon composite material is approximately 120° C. Accordingly, previously-proposed composite fan blades do not allow the tips of the blades to rub at all, and in doing so accept instead a reduction in the efficiency and an increase in engine fuel burn. There is therefore a need for a fan blade that can permit the tip of a composite fan blade to cut or rub into the liner and thereby maintain the fan efficiencies equivalent to existing all titanium fans.
The present disclosure seeks to provide a novel blade, which at least reduces the above problem whilst providing an effective cutting surface to the blade tip.